The goal of this proposal is to understand the mechanism of CTXphi integration and its relationship to chromosome dimer resolution. CTXphi_ is a filamentous bacteriophage that carries the genes for cholera toxin, the primary virulence factor of Vibrio cholerae. CTXphi integrates site-specifically into the chromosome of v. cholerae and thereby renders non-pathogenic isolates toxigenic. Unlike other characterized phages and viruses, CTXphi depends on chromosome-encoded recombinases, XerC and XerD, to integrate its DNA. Integration occurs near the chromosome mid-point in close proximity to the d/f site, which is the XerC and XerD substrate for resolution of chromosome dimers. Thus, the specific aims of this proposal include: I) definition of DNA sequences required for CTXphi integration and recombination at d/f, II) characterization of recombination by XerC and XerD in vitro; 111) identification, via a genetic screen, of additional chromosomal genes required for CTXd_ integration; and IV) characterization of the phage encoded rstB gene and examination of its effect on CTXphi integral ion. Results from these studies will broaden our understanding of CTXphi biology and the evolution of V. cholerae pathogenicity and will clarify the mechanism of a novel means of phage integration.